As you can read in the help file of the ashina data set (see help(ashina)), the variable grp indicates whether the placebo was given first or the active treatment was given first. In the long format ashina_long the measurements for placebo first and for active treatment first are arranged underneath each other, so it may be handy to have a variable period indicating when a particular observation (row) was done. Add this variable period to the ashina_long data frame (its possible values are either 1 or 2).

 

ashina_long
   grp  treat  vas id
1    1 active -167  1
2    1 active -127  2
3    1 active  -58  3
4    1 active -103  4
5    1 active  -35  5
6    1 active -164  6
7    1 active   -3  7
8    1 active   25  8
9    1 active  -61  9
10   1 active  -45 10
11   2 active  -38 11
12   2 active   29 12
13   2 active    2 13
14   2 active  -18 14
15   2 active  -74 15
16   2 active  -72 16
17   1   plac -102  1
18   1   plac  -39  2
19   1   plac   32  3
20   1   plac   28  4
21   1   plac   16  5
22   1   plac  -42  6
23   1   plac  -27  7
24   1   plac  -30  8
25   1   plac  -47  9
26   1   plac    8 10
27   2   plac   12 11
28   2   plac   11 12
29   2   plac   -9 13
30   2   plac   -1 14
31   2   plac    3 15
32   2   plac  -36 16

 

(from: Dalgaard, 2008, Introductory Statistics with R, Springer)